Conventional means for topical delivery of active ingredients to the skin of a recipient is mainly based on microneedle technology. There are two major limitations in the current microneedle's development: 1) in academic research, most published studies focused on the delivery of macromolecules (e.g. vaccines, insulin) using microneedles, in which many applications however are not explored especially in skin care; 2) On the other hand, microneedle therapy system (MTS) has recently become a fashionable beauty treatment, in which a device, usually in the form of a roller or stamp with many fine needles is used to create micro-channels in the skin prior to the further treatment. However, the increased risks of microbial infection due to the reused microneedles and biohazardous sharps left in the skin after use limit its applications and popularity. As such, a biodegradable microdepot system with the capacity of effective penetration while providing sustained active release in one single step could be an alternative to the conventional means based on the microneedle technology.
In U.S. Pat. No. 7,615,234, a method of delivering at least one therapeutic compound or a formulation comprising the same is disclosed, where a pioneer projectile in different dimensions and shapes is used in that method. Although the pioneer projectile in '234 appears to be biodegradable, a driving force is required to inject the therapeutic compound or formulation through the penetration of the projectile into the skin. It is not convenient for consumers who may not have enough skills to handle injection needle. There is also a risk of losing control when exerting too much force onto the therapeutic compound or formulation based on the method of '234. The injection mechanism for introducing the projectile to penetrate through the skin is somehow an invasive approach which may also be at risk of physically damaging skin tissue during the injection process.
In Henry et al. (1998), it disclosed a metallic microneedle assay which was pressed into epidermis using a force of about 10 N applied with a small wooden probe (2 mm in diameter; Baxter Healthcare, Round Lake, Ill.). To better simulate the in vivo mechanical environment, the dermis was placed below the epidermis as a supporting cushion. Insertion of the arrays into skin required only gentle pushing (estimated to be approximately 10 N, which is about the force needed to push an elevator button). After the microneedles were inserted, the epidermis and microneedles were inspected by light (StereoZoom 7; Bausch & Lomb, Rochester, N.Y.) and/or scanning electron (S-800, Hitachi, Tokyo, Japan) microscopy. FIG. 1 shows the microneedle tips inserted across epidermis. An array of microneedles was inserted into the stratum corneum side of human epidermis. The underside of the epidermis is shown, indicating that the microneedles penetrated across the tissue and that the tips were not damaged. Arrows indicate some of the microneedle tips. Inspection by light and electron microscopy showed that more than 95% of microneedles within an array pierced across the stratum corneum of the epidermis samples. However, such microneedle tips are invasive and difficult to handle by consumers in general. Fabrication thereof and in most conventional transdermal delivery means involve the use of acid which may cause side effect to the user such as allergy or actual skin damage.
To meet the need for a delivery system which is safe and reliable, an active ingredient-loaded transdermal delivery system with sufficient penetration efficiency while biodegradable at a certain time period is desired.